The field of hypobaric research for extending the storage life and preservation of food and horticultural commodities has been controversial since its inception more than 40 years ago. Since then, scientists expert in the art of post-harvest physiology have advanced numerous claims both commending and condemning hypobaric storage, and peer-reviewers have approved research publications promulgating both sides of the controversy with mutually incompatible opposing views.
Hypobaric storage was first attempted by Burg in 1966, and the concept received the 1979 US Food Technology Industrial Achievement award for ‘development of an outstanding food process or product that represents a significant advance in the application of food technology to food production’. However, shortly thereafter, and continuing to the present time, research publications from England, Holland, Canada, Japan, Israel, China, and the U.S. attributed numerous shortcomings to hypobaric low pressure storage technology (“LP Technology”).
For decades, LP Technology has been severely criticized as having implicit deficiencies. Multiple shortcomings were found with respect to severe desiccation, excessive production of ethylene (C2H4), inability to remove ethylene from within tissue, excessive fungal growth/decay, failure to ripen or improper ripening, and loss of flavor and aroma.
First, it was found that LP storage severely desiccated horticultural commodities, even though humidifiers were used to provide moisture within the low pressure chambers. Tests funded by NASA in connection with the Mars Greenhouse Program, resulted in desiccation of seedlings present in hypobaric light growth chambers within hours at a high RH and pressures as low as 20 mm Hg. At 75 mm Hg the plants sensed water stress even though their roots were immersed in a water supply and the humidity in their surroundings was 95% or higher (Paul et al., 2004). During one day the differential expression of more than 200 genes was altered. Genes associated with desiccation showed increased expression in hypobaria (low pressure) and were unaffected or repressed by hypoxia (low oxygen), while both hypobaria and hypoxia induced genes associated with fermentative pathways. Paul et al. concluded that a plant's response to hypoxia was a specific adaptation to perceived desiccation overlaid upon an adaptation to hypoxia. Second, it was reported that LP Technology was incapable of lowering internal levels of active ethylene (C2H4) and that the process damaged commodities, causing them to produce “wound” C2H4. Third, it was stated that the low pressure system's requirement for a high relative humidity condition should promote fungal growth and resultant decay of the commodities while in storage. Fourth, while LP Technology was recognized as an alternative method of producing a low oxygen environment, it was concluded that the technology did not provide or replace the benefit that added carbon dioxide produces in controlled atmosphere storage systems, and instead LP removed carbon dioxide.
Fifth, and equally troubling, is the claim that low pressure conditions evacuate and out-gas flavor and aroma components from fruits, resulting in poor flavor and unsatisfactory ripening. This hypothesis was first advanced by Wu et al. (1972) based on a study with green tomatoes. Apples also failed to develop normal flavor and aroma when they were ripened after prolonged LP storage (Bangerth, 1984), but when the same behavior was noted after apples were kept in CA for a prolonged time, it was suggested that this effect must be caused by O2 depletion rather than a low pressure. Bananas transferred to air after 4 months at 40-50 mm Hg (0.8 to 1% O2), ripen with normal color, texture, sweetness, flavor, and aroma, but after only 11 days in CA at 1% O2 they lose their ability to produce ethylene or develop acceptable flavor and sweetness. The same may be true of green tomatoes since the air change rate in Wu et als. LP apparatus was only 0.05 chamber volumes per hour, and therefore the fruits must have consumed nearly all of the supplied oxygen at the lower pressures tested.
Consequently, numerous barriers to effective LP Technology have been long-recognized and debated, and LP Technology has never been viewed as an improvement, let alone a substitute, for conventional controlled atmosphere storage systems. The holy grail of LP Technology is a system that achieves successful long-term physical storage in excess of what is possible using conventional controlled atmosphere systems, while also avoiding aesthetic damage to the flavor and aroma of the commodity. Such a system has evaded the food preservation industry for decades, and has been viewed as an impossibility based on many existing studies.
Water Loss
Perhaps the greatest hurdle to preserving the storage life of post-harvest commercial commodities is preventing water loss, and in that category, LP Technology has been a proven failure. Reports that LP Technology desiccated commodities so diminished interest in the process that by the early 1990's, hypobaric storage research essentially ceased in the western hemisphere. The excessive commodity weight loss caused by LP Technology (water loss/desiccation) has been confirmed as recently as 2008 by Chinese researchers.
According to the first law of thermodynamics, the quantity of water which can be evaporated from an adiabatic system depends on the amount of heat added. Water loss results in evaporative cooling, which lowers a stored commodity's temperature unless the latent energy used to change the state of water from liquid to vapor is replaced from a heat source. Accordingly, commodity water loss in a refrigerated room utilizes latent heat from respiration and fermentation, sometimes augmented or reduced by additional heat transferred by convection and/or radiation to or from the stored product from the environment. When the commodity remains at a constant temperature, if the heat needed to evaporate the transpired water is less than the respiratory and fermentative heat, the commodity must transfer heat to its environment, but if the heat used to transpire water exceeds the respiratory and fermentative heat, the commodity is acquiring heat from its environment. A commodity stored in a refrigerated space cannot remain at a constant temperature and loses more water than its respiratory heat is capable of vaporizing unless it is colder than its environment and receiving heat from it.
Researchers have examined why, in so many studies, LP Technology desiccated commodities. Commencing in 1978, it was suggested that leak rates may have an effect, and it was theorized that the leaks allow non-humidified air at atmospheric pressure to enter chambers, making it difficult to increase the humidity to the level required to prevent excessive commodity water loss.
However, the leakage hypothesis contradicted findings from numerous published studies. (Tolle, W. E. (1969) Hypobaric Storage of Mature Green Tomatoes. USDA Agr. Research Rept. 842, pp. 1-9; Tolle, W. E. (1972) Hypobaric Storage of Fresh Produce. 1972 Yearbook. United Fresh Fruit & Vegetable Association. pp. 27, 28, 30, 34, 36, 43.) Tolle attempted to prevent commodity water loss by automatically controlling the humidity responsive to a humidistat installed in the storage chamber. In Tolle's apparatus, strawberry weight loss increased at higher flow rates even though the electro-sensors recorded the same high humidity independent of air-flow. When tomatoes were stored in his apparatus without humidification, water droplets condensed as beads inside the lids of the storage chamber. In both instances, the stored commodity was saturating the rarified air. In addition, Lougheed et al, (1977) LPS—Great expectations. In: Dewey, D. H. (ed.) Horticultural Report: Controlled Atmospheres for the Storage and Transport of Perishable Agricultural Commodities, pp. 3-44, reported that apples rapidly desiccated even when a humidity sensor indicated that the humidity in the chamber was close to 100%. Modern Chinese hypobaric storage systems experience this same difficulty, and they also utilize a humidistat to control the chamber's relative humidity. These findings seemed to contradict the hypothesis that an excessive vacuum leak rate is a primary cause of desiccation during hypobaric storage.
This controversy was partially resolved by proprietary tests carried out in a prototype hypobaric intermodal container, in which a highly accurate bureau of standards mirrored dew-point sensor monitored and controlled the humidity. Whenever the relative humidity decreased below 95%, the sensor energized a water boiler's electric immersion heater causing low-pressure cold-steam to be injected into the rarified air-change before it entered the storage chamber. Without cargo present, the system worked as envisioned, but in the presence of a full commodity load the humidification heater failed to energize because the dew-point sensor could not distinguish whether the chamber moisture had originated from the commodity or the boiler. This test demonstrated that humidity could not be accurately controlled by a humidistat installed within a hypobaric storage chamber filled with commodity.
Leakage is not the only experimental error that can cause excessive commodity weight loss during hypobaric storage. A study comparing CA and LP storage of green peppers illustrates the error created by humidifying the air-change at atmospheric pressure before it enters an LP chamber (Hughes et al., 1981). Peppers kept in LP at various pressures became ‘wrinkled and flaccid, showing severe desiccation’, losing weight 4-5 times faster than those stored in CA. LP was judged to be the least effective method of storing green peppers, and it was concluded that ‘water loss thus appears to be a major problem in hypobaric storage’. The humidified atmospheric air had expanded and decreased in RH as it entered the LP chamber. Storage at close to saturation in CA had been compared to flowing a 5-20% RH air-change in LP. The hypobaric method preserves green peppers without significant weight loss when the air-change is saturated at a low pressure rather than atmospheric pressure prior to entering the storage chamber. This same experimental mistake is evident in studies carried out at the National Vegetable Research Station in England, and by Wu et al. (1972) with potatoes, apricots, peaches, sweet cherries, apples, and tomatoes, and by Ilangantileke and Salokhe (2006) with mangoes.
Ethylene Buildup
Another claimed shortcoming of LP Technology is an inability to remove “active” ethylene (C2H4) from within commodities during storage. Ethylene is a natural plant hormone that shortens storage and shelf life by hastening fruit ripening and the senescence of flowers and vegetables. Horticultural products also produce this gas when they are subjected to stress or microbial infection during shipping, handling, or storage. Ethylene production poses one of the greatest threats (and associated costs) to horticultural harvesting, transport, and storage world-wide. Regardless of whether tomatoes were stored at atmospheric pressure (=20.9% O2) or in pure O2 at ⅕ atmosphere pressure (=20.9% O2) they ripened at the same rate. Stenvers and Bruinsma (1975) concluded that ‘Ethylene produced in ripening tissue may well exert its physiological effect during passage, within the cell, from the site of biosynthesis to the intercellular space, and reduction in the intercellular ethylene content by low atmospheric pressure is not effective in regulating senescence’. This experiment was carried out in a sealed LP system which accumulated at least 1-2 ppm ethylene prior to each daily chamber ventilation and re-evacuation. It was ‘assumed’ that 5 μl/l ethylene is required to stimulate ripening, but harvested tomatoes respond to one-thousandth that concentration, and 1 ppm is a supra-optimal dose. Daily ethylene accumulation had caused all of the tomato fruits to ripen at the same rate.
Carbon Dioxide Removal
Researchers also have claimed that LP Technology has no advantage over controlled atmosphere storage, and is in fact less effective than CA because LP cannot elevate CO2 and instead lowers the gas's internal concentration within commodities by enhancing its diffusive escape (see FIG. 7).
Hypobaric System Shortcomings
Early hypobaric systems were designed to maintain a pressure range between 80 and 150 mm Hg in order to prevent low-oxygen injury to the commodity. At the low end of this range, 80 mm Hg, the oxygen levels were approximately 2%. (For purposes of this application, to simplify comparisons between atmospheric and sub-atmospheric pressure, gas and vapor concentrations are expressed as a percent gas or vapor; 2% oxygen thus refers to a partial pressure of 0.02 atmospheres). At oxygen levels lower than 2%, the end-products of fermentation, such as ethanol, acetaldehyde and ethyl-acetate, were expected to accumulate within the commodity and eventually reach toxic concentrations that cause off-flavors, necrotic and discolored tissues, off-odors and other symptoms of low-oxygen damage and spoilage. Thus, controls were added to the early hypobaric systems to prevent the development of vacuums lower than 80 mm Hg, thereby ensuring oxygen levels above 2% at all times.
Later, it was found that during hypobaric storage the tissues could withstand lower levels of oxygen if the air was humidified at close to 100%. Research was done with systems that brought pressures down to 10 mm Hg, and a corresponding 0.13% oxygen content at 0° C., and 15 mm Hg at 10° C. (0.1% O2), and 20 mm Hg at 13° C. (0.24% O2) using humidifiers to keep the air saturated. Horticultural commodities were never tested at pressures lower than 10 to 15 mm Hg because researchers were aware that to avoid low oxygen damage when a totally anaerobic condition was approached would entail accurately and reliably supplying less than 0.1% oxygen, which was not possible to accomplish using available pressure controllers, measuring devices and temperature responsive dynamic pressure control.
Such devices were not required to store meat hypobarically since to accomplish this, the chamber only needed to be sealed to prevent air changes from entering, cooled to −1° C., and continuously evacuated. Meat does not suffer low oxygen damage when the vapor pressure of water in the meat is reached at 4.6 mm Hg. This prevents a further pressure decrease since the meat has begun to gently ‘boil’ at −1° C. At that time, the storage chamber's atmosphere is entirely composed of cold steam which is continuously being evacuated from the chamber and replaced by fresh cold steam evaporated from the meat. The cold-steam flushes away odors and any in-leaking oxygen, thereby maintaining the totally anaeroic condition required for maximum meat storage, while causing a tolerable weight loss during several months. Prior hypobaric storage patents accurately claimed 4.6 mm Hg at −1° C. as the preferred condition for meat storage, but they could not, and did not, verifiably claim this as a lower limit for horticultural commodities.
Unfortunately, while the problems of low oxygen injury were being diminished with combined low pressure between 10 and 20 mm Hg and the advanced humidifying systems, researchers found that the commodities often were still experiencing desiccation and other problems, rendering the systems not useful for extended storage purposes.
In the 1970's thru recent times, hypobaric systems were produced that could reduce internal pressures to below 10 mm Hg. However, leakage into these systems, including modern systems of today, was high—from under 10 mm Hg per hour to in excess of 30 mm Hg per. Researchers were aware that if the pressure was reduced to the vapor pressure of water at the storage temperature (see Table 1, infra), there would be no oxygen present in the storage chamber, and the commodity would be rapidly and irretrievably damaged by the anaerobic environment. By staying at or above 10 mm Hg at 0° C., and 20 mm Hg at 13° C., researchers were able to prevent the pressure from accidentally dipping low enough to cause low oxygen damage. However, even when assuring these low pressures, the research led to desiccated commodities.
Leak rates, lack of pressure control, lack of management of internal respiratory heat build-up, lack of chamber temperature uniformity, increases in internal ethylene, and problems with packaging have contributed to an inability to mechanically achieve the requisite correlated conditions to achieve vastly extended storage of fresh horticultural commodities. Furthermore, the danger of approaching certain conditions without proper control has served as a deterrent.
Accordingly, there is a decades-long un-met need in the art for hypobaric systems and methods that solve not just one or two, but all of the problems associated with LP Technology. The need is for systems and methods to extend storage and preservation for food and horticultural commodities that, among other benefits, simultaneously: 1) retain sufficient water within the cellular tissues; 2) prevent buildup of C2H4 and related harmful toxins in the tissues; 3) prevent fungal and bacterial growth; 4) retain flavor components and/or aroma molecules; 5) avoid low oxygen or high carbon dioxide damage and physiological disorders; 6) prevent geotropic and epinastic responses; 7) prevent ethylene responses such as ripening, senescence, de-greening and floral fading; and, 8) provide storage periods longer, and preferably significantly longer, than conventional controlled atmosphere storage systems